1. Field of the Present Invention
The present invention is in the field of printer systems and more particularly, printer systems employing multiple beams.
2. History of Related Art
A halftone image is a reproduction of a contone image (e.g., a photograph), using spots of varying size to produce apparent shades of gray. The darker the shade at a particular point in the image, the larger the corresponding spot in the halftone. In traditional publishing, halftones are created by photographing an image through a screen. In desktop publishing, each halftone spot is created by a collection of darkened pixels printed by a laser printer or digital image setter. In the case where the halftone spots form a regular grid the frequency of the halftone dots is measured in lines per inch. Higher printer resolution enables effective use of higher density of halftone dots, enhancing image quality.
An image typically is an array of values created by scanning a continuous tone original such as a photograph. The scan results in light striking a photosensitive device that produces an electrical signal having a value determined by the characteristics of the reflected light. The value produced by the photosensitive device is converted to a digital value that represents the tonality of the original image. The photosensitive device produces an array of such values. This array of values is sometimes referred to as the original image.
One common method of creating a halftone image is achieved by using an array of digital values, typically ranging from 0 to 255, referred to as a threshold array. The threshold array represents one or more halftone cells. First, the threshold array is tiled across the original image, with or without an offset between adjacent rows of the tiles. The gray levels of the original image are then compared with the values of the corresponding tiled array positions and blackened if they are lower, or left unprinted if they are higher. In this way the original image is converted into an image with halftone “dots”. The blackened picture elements (pels) can then be printed using a binary printer. Other methods also exist to create halftones such as algorithmic error diffusion or direct binary search methods. The threshold array method is one of the most common because it requires minimal computational processing.
Thus, the halftone dots are formed by thresholding the values of the original image with the tiled threshold array. The threshold array itself may be generated by a spot function based on a mathematical algorithm. The spot function determines a dot's shape as it grows in size from nonexistence (white) to solid (black). A variety of spot functions exist, including round, elliptical and diamond. Straight-line and star-shaped spot functions are also possible, as are numerous other variations.
A multiple beam printer, as suggested by its name, is characterized by a raster having two or more laser beams operating simultaneously. Multiple beam printers often produce halftone images exhibiting banding. Banding is also frequently referred to as moire banding. Banding is an undesirable periodic artifact which the human eye detects and which is well known in the field of printing systems. Banding in multiple beam printers is affected by differences in the power of individual laser beams, scan direction beam offsets, and spot spacing errors. Unfortunately, the spatial frequency for banding is often near the most sensitive region of human perception and, accordingly, very small errors of any of these parameters can result in easily detectable banding. It is desirable to implement a multiple beam printing system that minimizes banding effects.